Electric-field structuring of composite materials

ABSTRACT

A method and apparatus for the spatial distribution of a filler component within the matrix of a composite material. An electric field is applied to a mixture consisting of two or more components that have different dielectric permittivities, at least one of which is initially in the form of a liquid polymer or pre-polymer. An alternating electric field is established between a pair of electrical conductors or electrodes that are moved relative to each other, so as to alter the spatial intensity of the electric field in a controlled manner. Polarized particles or elements of the filler component that are coupled to a moving electrode by a dielectrophoretic force can consequently be dragged into a predetermined spatial configuration. As the size of the inter-electrode gap is changed, the applied voltage can be adjusted in order to maintain electrically induced polarization of the filler component.

The present invention relates to a method and apparatus used to exercisecontrol over the spatial distribution of a first component within asecond component of a composite material.

BACKGROUND OF THE INVENTION

Many composite materials are commercially available which consist of asecond component, for example a filler, distributed within a firstcomponent, referred to as the matrix material. In general terms, thefiller component comprises one or more materials having mechanical,thermal, electrical or magnetic properties that differ significantlyfrom those of the matrix material. When the components are combined, acomposite material is formed that has physical properties unlike thoseof either the filler or the matrix. The properties of the compositematerial can be varied and controlled by changing the relative amountsor volume fractions of the components. If the filler component israndomly distributed within the matrix then, typically, the physicalproperties of the composite are independent of the orientation of onecomponent within the other of the material. Such a material is said tohave isotropic properties. Alternatively, if the spatial distribution ofthe filler component is controlled, then it is possible to makecomposites having physical properties that vary according to theorientation of the material. Structural fibreglass composites are a goodexample of the latter. The glass fibres can be deliberately aligned,parallel to the direction of an applied force, in order to takeadvantage of the relatively high tensile strength and stiffness thatthey have in comparison to the matrix. This composite is stronger in onedirection and is said to have anisotropic properties.

In ‘Connectivity and Piezoelectric-Pyroelectric Composites’ Mat. Res.Bull. 13 pp525–536 (1978), Newnham R. E et al, the descriptive termconnectivity was used to classify the distribution of the constituentsin composite materials. An index number is assigned to each componentaccording to the number of dimensions in which it is physicallyself-connected. A thin rod is said to be self-connected in one dimensionand no other. A continuous matrix is self-connected in three dimensions.Using this terminology, a composite consisting of an array of alignedrods of filler material held by a continuous matrix is referred to as a1-3 composite. Where individual filler particles are uniformly dispersedin a continuous matrix, they are said to be physically self-connected inno dimension. This type of material is referred to as a 0-3 composite.

Piezoelectric composite materials, consisting of a ferroelectric ceramicin an electrically-inactive polymer matrix, provide an example wherebythe spatial distribution of the second component has a fundamentalinfluence on physical properties. Both 0-3 type and 1-3 type compositesare routinely used for passive sensors. The 0-3 type have adequatesensitivity and have a cost advantage due to their simplicity ofmanufacture. Their greater flexibility and formability make them thepreferred choice for large area applications such as sonar. Compositeswith an 1-3 type connectivity are far more sensitive, but are also moreexpensive to make. They are preferred for active devices and forarray-type sensors. Piezoelectric arrays are used for such applicationsas acoustic imaging and for medical ultrasound.

In array-type composites or 1-3 type composites, two of the key factorsaffecting properties are the size of the individual elements of thesecond component and the distance between those elements, referred to asthe periodicity. In production, steps must be taken to ensure that thesefactors are controllable within specified limits. A commonly usedtechnique utilises precision micro-machining to accurately cutindividual array elements from a solid ceramic block. Arrays have alsobeen made by injecting ceramic powder, or powder in suspension, into amould under pressure. The resulting pre-form of the composite structuremust then be sintered to consolidate the ceramic. In an alternativemethod, rods or ‘fibres’ can be aligned mechanically, by hand orotherwise, to produce a desired aligned structure. In practice this isdifficult to achieve with the required precision due to the small scaleof operations. Typically, the useful diameter of the rods lies in therange 30 to 400 microns. Once the desired structure of elements of thesecond component has been created it can then be bonded or embedded in apolymeric matrix to complete the composite material. Most commonly, aliquid pre-polymer is allowed to permeate the aligned structure andsubsequently solidify. The term pre-polymer refers here to a compound ormixture of compounds that can undergo a chemical reaction to produce apolymeric solid. Alternatively, it is conceivable that a molten polymercould be allowed to permeate the aligned structure and then to solidify.Surface-active ‘coupling agents’ can be used to improve the usefulproperties of the composite by chemically modifying the interfacebetween the filler and the matrix components.

In contrast, the production of 0-3 type composites is morestraightforward. The filler or second component, in powder form, ismixed intimately with the first component, a liquid polymer orpre-polymer. The liquid wets the filler particles, before being made topolymerize or otherwise solidify. High volume fractions of filler areused and the composite is shaped by hot-pressing or warm-rolling.Ostensibly, a commercial 0-3 type composite consists of individualfiller particles, each one being completely surrounded by a layer of thematrix material. In practice, the action of rolling or pressing oftenbrings the particles into such close proximity that direct electricalcontact may occur.

One method that could be used to induce a predetermined spatialdistribution of the filler component in composites is referred to asdielectrophoretic assembly or electric-field structuring. In thisprocess, a dispersion of filler particles in a liquid polymer orpre-polymer is exposed to a moderate a.c. electric field. Under suitableconditions, the filler particles become polarized and exhibit a mutuallyattractive force, which causes them to form chain-like structuresbetween the electrodes. The liquid is then solidified by means of achemical reaction or a change in temperature and the newly-formedstructures are thereby fixed in place to form a composite material withanisotropic properties. This method has the potential advantage thatmaterials having the sensitivity of 1-3 type composites could be made,whilst retaining some of the simplicity of the manufacture of 0-3 typecomposites.

The electric-field structuring technique utilises this dielectrophoreticforce, which is responsible for an electrorheological effect. This isdiscussed in ‘Induced Fibrillation of Suspensions’. Journal of AppliedPhysics 20 pp 1137–1140 (December 1949), Winslow W. M. and‘Dielectrophoresis: The Behaviour of Neutral Matter in Non-UniformElectric Fields’. Cxnbridge University Press (1978), Pohl H. A. Variousparameters affecting this are: the dielectrophoretic or polarizationforce, which is directed to produce the desired particle structure;viscous drag in the fluid, which resists particle motion; andsedimentation, which must be controlled. Alternating electric fields areused, by preference, to avoid electrophoresis. Applied electric-fieldstrength is deliberately moderated to suppress such effects aselectrically-induced turbulence in the fluid and accelerated curing ofthe polymer. Applied electric-field frequency is dictated by thedielectric properties of the fluid and the filler.

One of the major pitfalls associated with the electric-field structuringtechnique is sedimentation. Where particles of the filler component havea higher density than the surrounding fluid component, then they willfall out of suspension under the influence of gravity. The rate ofsedimentation depends on particle size and shape and also on theviscosity of the surrounding fluid. In practice, the magnitude of thedielectrophoretic force can be set to overshadow viscous drag and alsothe effect of gravity. However, the forces acting on different sizedparticles are not of the same magnitude. This makes precise control overthe shape of the electric-field-induced structures difficult andirregularities commonly occur. A further difficulty concerns theviscosity of the surrounding fluid, which is not constant over thecourse of the processing cycle. For example, thermosetting polymers suchas epoxy resins exhibit a progressive increase in viscosity with time aspolymerization proceeds. At the same time, the polymerization reactionitself is exothermic and generates heat. The fluid experiences a rise intemperature and consequently its viscosity decreases. Furthermore, therate of reaction is increased at the higher temperature. These competingeffects make precise control over fluid viscosity difficult to achieve.Accordingly, the rate of sedimentation of suspended particles is oftenuncertain.

In common with many composites, the interface between the filler andmatrix components has a controlling influence on the physical propertiesof materials produced by electric-field structuring. The surfaceelectrical properties of the filler particles, in particular, are ofprime importance. Normally particles are completely surrounded by alayer of adsorbed polymer, giving true random 0-3 type connectivitywithin the matrix. Where these layers form insulating barriers betweenthe particles, the useful electrical properties of the compositematerial can be adversely affected. Furthermore, where variations in thesizes of individual particles exist, chain branching in the fieldinduced structure is found to occur. Anisotropy in the electricalproperties will then be a function not only of the amount of filler, butalso of the degree of this chain branching. In practice, some disparityin the spatial distribution of the filler particles is found to occurbetween otherwise identical composite samples. Hence, significantvariability in the physical properties of composites prepared byelectric-field structuring can normally be expected.

BRIEF SUMMARY OF THE INVENTION

This present invention is directed towards a method and apparatus formanufacturing a composite material which overcomes the problem ofsedimentation, hitherto associated with the electric-field structuringof composite materials. A further objective is to provide a means ofimproving the useful physical properties of such materials bycontrolling the spatial distribution of the filler component within thematrix.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, and withreference to the figures, of which:

FIG. 1—is a schematic elevation illustrating an apparatus according toan embodiment of the present invention, immediately before an electricfield is applied.

FIG. 2 a—is a view of the FIG. 1 embodiment after an electric field isapplied.

FIG. 2 b—is a view of the FIG. 1 embodiment as the second electrode iswithdrawn. Thus it shows a stage in the processing sequence followingthat shown in FIG. 2 a.

FIG. 2 c—is a view of the FIG. 1 embodiment as the second electrode iswithdrawn through the template. Thus it shows a stage in the processingsequence following that shown in FIG. 2 b.

FIG. 3—is a schematic elevation illustrating an embodiment having amodification to the apparatus, immediately before an electric field isapplied. The apparatus is similar to that shown in FIG. 1 with theaddition of a device to pre-locate the filler particles as they rest onthe base electrode.

FIG. 4—is a schematic elevation illustrating a further embodiment of thepresent invention, immediately before an electric field is applied,having the addition of a device to pre-locate quantities of a secondfluid resting on or adjacent to the base electrode.

FIG. 5—is a schematic elevation illustrating an apparatus according tothe present invention, immediately before an electric field is applied.The apparatus is similar to that shown in FIG. 1 but the template isabsent.

According to a first aspect, the invention comprises a method for themanufacture of composite materials including the steps of:

(a) adding a first composite component, comprising at least onenon-conducting or weakly conducting fluid, to a receptacle;

(b) adding a second composite component, comprising at least one solid,liquid or gas capable of polarisation; whereby steps (a) and (b) can becarried out in either order;

(c) applying an alternating electric field across two electrodesimmersed in the resultant mixture and wherein the two electrodes aremoved relative to each other during the formation of the composite.

The first component and the second component have different dielectricpermittivities as the first component is non-conducting or weaklyconducting and the second component is capable of polarisation. Undersuitable conditions, a dielectrophoretic force is induced by an appliedelectric field such that there is a mutual physical attraction betweenindividual elements of the second component and also between elements ofthe second component and the electrodes. When one or more of theelectrodes are physically moved relative to each other elements of thesecond component are dragged along, under the influence of thedielectrophoretic force, and can be redistributed within the surroundingfluid. Preferably, one of the electrodes is located in a fixed positionat the base of the receptacle.

Preferably, means to pre-locate the second component within the firstcomponent prior to applying the electric field is provided. This meansmay be positioned at the base of the receptacle or between the twoelectrodes, immersed in the first component and may comprise a devicewhich assists in the spatial redistribution of the second component. Anexample of such a device at the base of the receptacle could comprise anirregularity in the base surface of the receptacle.

Preferably, the second component is drawn through holes within atemplate during the relative movement between the two electrodes. Thepattern of the holes within the template corresponds to the requiredpattern of the second component within the first component. This patternis a function of the property requirements of the composite material.

Preferably, the second component contains more than one type of solidparticle or filler. A further filler may comprise differently shapedparticles of the same electrically polarizable material as the otherfiller or may be another electrically polarizable or non-polarizablematerial.

The second component may also comprise one or more types of liquiddispersed in the form of droplets or globules. On exposure to anelectric field of suitable strength and frequency, polarized droplets ofthe second component form. If the second component were present asdroplets or globules, these may coalesce into larger globules. If theconductivity of the second component exceeds that of the firstcomponent, the globules can be drawn out into elongated columnarstructures by physical movement of the electrodes. Elongation isparallel to the direction of the applied field and the spatialdistribution of the columns is controlled by the electrodeconfiguration. As before, the newly-formed structures may be fixed inplace by solidification or curing of the fluids concerned.Alternatively, if only the first component is solidified or cured then asecond component can subsequently be removed to leave a series or anarray of holes. The apparatus used in this embodiment may benefit from atemplate located between the electrodes as previously described. Avariation of this technique exists whereby the dielectrophoretic forceis used to draw columns of the second component, a miscible fluid,against the pull of gravity, from a pool of such liquid adjacent to thebase electrode. A further variation of the technique exists when theconductivity of the second component is less than that of the firstcomponent. In this case, globules form that are flattened in a planeperpendicular to the direction of the applied field. Physical movementof the electrodes is utilized, as before, to dictate the spatialredistribution of the second fluid within base fluid before one or bothare sofidified.

Preferably, the second component includes electrically-polarizable rodsor fibres. These may be aligned within the first component achieving apredetermined spatial distribution. The length of the rods can be chosenso that they completely span the inter-electrode gap. Composites made inthis way have no insulating barriers between the ends of the rods so theelectrical properties of the second component can be fully utilised andtrue 1-3 type connectivity is achieved. Equally, composites made in thisway can be sectioned after assembly so as to achieve this effect. Thisaspect of the invention is intended to preclude any adverse effect onthe physical properties of the composite material that could result fromthe presence of barrier layers of polymer, interspersed between the endsof particles of the filler component.

A second aspect of this present invention is an apparatus for themanufacture of composite materials with controlled spatial distributioncomprising a receptacle capable of containing a fluid and two electrodescapable of relative movement whereby the first electrode is positionedat the base of the receptacle. The electrodes may comprise a series orarray of electrically connected protrusions or needle-like conductors,but may equally comprise any other single shaped conductor or aplurality of such conductors such as flat plats, spheroids, cylinders,cones, tubes, carbon fibres. The configuration and the shape of theelectrodes themselves then determine the location and the periodicity ofthe particle chains. One or both of the electrodes may be wholly orpartially covered by an electrically insulating material.

The apparatus preferably further comprises a template containing atleast one hole and more preferably a series or an array of holes. Inuse, this is immersed in the first component.

In a preferred embodiment, the first component and the second componenteach comprise at least one liquid either as a heterogeneous mixture ofimmiscible fluids or an emulsion. In this embodiment, the secondcomponent is dispersed in the form of droplets and the first componentis a liquid polymer or liquid pre-polymer.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a chamber (10) contains a non-conducting or weaklyconducting fluid (12) which is a pre-polymer of Epotek 302-3M (availablefrom Epoxy Technologies, Bdlerica, Mass.,U.S.A.) and is the firstcomponent of the composite. The chamber also contains a supply ofpolarizable particles (11) in the form of short rods of PZT5A (availablefrom CeraNova Corp., Fr Mass., U.S.A.) the second component of thecomposite. For clarity, the particles are depicted as being elongatedrods. The particles are resting on a first electrode (20), which is madefrom any electrically conductive material. The fluid is an opticallyclear, two-part epoxy resin. Part A, which is based on diglycidyl etherof bisphenol-A (DGEBA) is mixed with Part B, which is a multi-functionalaliphatic amine, according to the manufacturers' recommendation.Dissolved air is then removed by placing the mixture in a reducedpressure environment for around 30 minutes, at room temperature. A smallincrease in temperature, to 30° C., will facilitate this operationalthough it must be noted that the rate of the polymerization reactionis increased at higher temperature. The fluid is then transferred to theprocessing chamber. The rods are 130 microns in diameter and are ofuniform length of 5 mm. (The diameter and length of the rods and theirpositioning is taken to be selectable in accordance with the intendedapplication of the composite).

A template (30) is positioned some distance above the base electrode andis fully immersed in the fluid. The template is made from anelectrically insulating material. In the template are a series or arrayof holes. Each hole has sides parallel in the plane of the figure andcan have any desired shape in cross-section. Where the holes arecircular in cross-section then they have a nominal diameter (d). Thedistance between the hole-centres is referred to as the periodicity (p).

A second electrode in the form of an array or series of needle-likestructures (21-24) interconnected to form one integral electrode,protrudes through the holes in the template and into the fluid below thetemplate. There is no restriction on the number of individual needlesthat may be used. Any of the individual needles or the first electrodeor may not be covered totally or partially by a layer of an electricallyinsulating material as required. The inter-electrode gap (g) is theshortest distance between the second electrode and the first electrodes.The second electrode and the first electrode are connected to an a.c.power supply (50), which is controllable in respect of output voltageand frequency. A switch (51) is shown in the electrical circuit. Controlover the electrical circuit is achieved by means of an externalcomputer.

Referring to FIG. 2 a, with the switch (51) closed the electricalcircuit is energised and an a.c. electric field of a few volts permillimeter, at a frequency of 3 kHz, is applied across theinter-electrode gap. The electric field strength is then graduallyincreased so that the particles become polarized (11*) and adielectrophoretic force is established between individual particles andbetween the particles and the electrodes. When the electric fieldstrength exceeds around 250Vmm⁻¹, the particles move physically underthe influence of the dielectrophoretic force and form prototypechain-like structures spanning the inter-electrode gap.

Referring to FIG. 2 b, with the electric field applied theinter-electrode gap (g′) is increased by moving the second electrode.The prototype chains of polarized particles (11*) remain coupled to theupper electrode structure by the dielectrophoretic force and are drawnalong with it against the pull of gravity. The polarized particlescooperative such that the chain-like structures become more elongated inthe direction of the applied field and localized where the fieldintensity is greatest. As the size of the inter-electrode gap increases,the chain-like structures are augmented by additional polarizedparticles that are drawn dielectrophoretically from the supply. Andthus, the chains of particles increase in length. The potentialdifference (r.m.s.) between the electrodes can be increased tocompensate for the increase in the number of particles on the chain tomaintain adequate polarization of the particles. The rate of widening ofthe inter-electrode gap is limited to around 10 mm per minute. If therate of widening is too fast then small fluid-filled gaps can developbetween the polarized particles. The high electric field that pertainsacross such a gap can easily exceed the dielectric breakdown strength ofthe fluid. Where this occurs then small gas bubbles form that can causeunwanted porosity in the composite.

Referring to FIG. 2 c, the size of the inter-electrode gap has beenfurther increased (g″). Particle chains are drawn into holes located inthe template. The size, shape and location of the holes are used tomould the particle chains into the required configuration. The chainsmay be further drawn out, through the holes, to any required length.Where there are insufficient particles available, the chains may notcompletely span the inter-electrode gap and may become detached from oneor other electrode. Once the required length of chain has been drawnout, the pre-polymer is cured for a period of time at an elevatedtemperature. It must be noted that increasing the temperature of theresin changes its electrical properties. An intermediate increase intemperature to around 35° C. for 45 minutes could be used if needed. Inparticular, electrical conductivity is increased and this can affectpolarization of the particles. A controlled external pressure may beapplied to the fluid as an aid to processing. Externally appliedpressure is commonly adopted in polymer processing as a means ofrestricting the formation of voids, which result from the presence oftrapped gases or vapour. The resulting structured-composite material canbe sectioned as required.

The present invention is not limited to the second component havingparticles of this shape. The switch for the electrical circuit may be amanual device or equally may be electronic.

Referring to FIG. 3, a further embodiment of the present invention isshown whereby a device (31) is added to the apparatus referred to inFIG. 1 that enables particles of a filler component to be pre-located onthe base electrode as an aid to processing.

Referring to FIG. 4, a further embodiment of the present invention isshown whereby a device (31) is added to the apparatus referred to inFIG. 1 that enables quantities of the second component as a fluid (13)to be pre-located on or adjacent to the base electrode.

Referring to FIG. 5, a further embodiment of the present invention isshown whereby the apparatus referred to in FIG. 1 can be used without atemplate situated intermediate to the electrodes. Physical movement ofthe electrodes is utilized, as before, to dictate the spatialredistribution of the finer component, but the precise control providedby the template is absent. The embodiments referred to in FIGS. 3 and 4can also be practiced in this way.

The electric-field structured composite material can be sectioned asrequired and its thickness can be tailored by lapping or by grinding. Auseful composite material, produced by this method, may incorporate thetemplate, where particles of the finer component are fixed inside theseries or array of holes. Otherwise, the useful material may be from theregion above the template and comprise a series or array of particlechains held in a matrix. The location and periodicity of the chains isdetermined by the positioning of the template holes. In such a way thespatial distribution of the filler component within the matrix can becontrolled.

It is within the scope of the present invention that the apparatus andmethods described above can be used in conjunction with firstcomponents, such as liquid polymers or pre-polymers, containing morethan one type of polarizable material. Useful materials for the secondcomponent include ferroelectric ceramics such lead zirconate titanate(PZT) and its derivatives, lead titanate, calcium-modified leadtitanate, relax or ferroelectric ceramics, electrostrictive materials,electrical conductors such as carbon fibre, graphite, metals, electricalsemiconductors and conductive organic polymers such as polyaniline,silicon carbide, silicon nitride, glass microspheres, glass fibres andalumina. Surface modification of the second component in the form ofconductive or dielectric coatings can be used to enhance thepolarizability of such materials for use with the apparatus and methodsfeatured in the present invention. Furthermore, non-polarizable andweakly polarizable materials can also be included where these wouldimpart useful properties to the composite.

Correct selection of the applied field frequency is critical to thesuccess of the electric-field structuring technique. A first estimate ofthe frequency (f_(s)) at which chain-like structures form can be derivedfrom the mathematical relationship:

$f_{s} > \frac{\left( {\sigma_{p} + {2\sigma_{f}}} \right)}{\left( {ɛ_{p} + ɛ_{f}} \right) \cdot \pi}$

In this relationship, σ_(p) and σ_(f) denote the effective (ie.frequency dependent) conductivity of the particles and the fluidrespectively; ε_(p) and ε_(f) denote the effective dielectricpermittivity of the particles and the fluid respectively. Ifconductivity is expressed in Siemens per meter (Sm⁻¹) and dielectricpermittivity is expressed in Farads per meter (Fm⁻¹) then the requiredfrequency is given in cycles per second (Hz). A more precise estimate ofthe required field frequency can be gained through consideration of thecomplex polarizability parameter. The required field frequency (and alsofield strength) can also be determined experimentally through directvisual observation of the particles and fluid as the applied electricfield conditions are varied. As a general rule, the effect of varyingthe applied field frequency should be assessed for each of thecomponents present.

It is to be understood that the embodiments described above areillustrative of the principles of the present invention and that otherembodiments and modifications of the invention may be readily devised inthe light of this disclosure whilst still remaining within the scope ofthe invention disclosed herein.

1. A method for manufacture of a composite material-comprising a fillermaterial distributed in a matrix material, the method including thesteps of: (a) adding a matrix composite component, comprising at leastone non-conducting or weakly conducting fluid, to a receptacle; (b)adding a filler composite component, comprising at least one solid,liquid or gas capable of polarisation; wherein steps (a) and (b) arecarried out in either order, resulting in a mixture of said components;(c) applying an alternating electric field across two electrodesimmersed in the resultant mixture and wherein the two electrodes aremoved relative to each other during the formation of the compositematerial.
 2. A method according to claim 1 wherein one of the electrodesis located in a fixed position at a base of the receptacle.
 3. A methodaccording to claim 1 including providing means to pre-locate said fillercomponent prior to applying the electric field, said means beingimmersed in the receptacle.
 4. A method according to claim 1 includingproviding a template comprising at least one hole between the twoelectrodes.
 5. A method according to claim 1 including means forapplying an external pressure.
 6. A method according to claim 1 whereinthe matrix component is a polymerizable liquid.
 7. A method according toclaim 1 wherein the filler component consists of rods, filaments orfibers of ceramic, carbon, metal, semiconductor, glass, polymer.
 8. Amethod according to claim 1 wherein the matrix component is a liquidpre-polymer of epoxy resin and the filler component consists of rods,filaments or fibers of lead zirconate titanate (PZT).
 9. A methodaccording to claim 1, in which method there is used apparatus comprisingthe receptacle capable of containing the fluid and the two electrodes,the two electrodes being separated by an inter-electrode gap being ashortest distance between the electrodes and being capable of relativemovement to widen the inter-electrode gap, whereby one of the electrodesis located in a fixed position at a base of the receptacle to form abase electrode.